The formation and development of latent images on the surface of photoconductive materials using liquid toner, the LEP process, is well known. The basic process involves placing a uniform electrostatic charge on a photo imaging plate (“PIP”) and exposing the PIP to a light and shadow image or to a scanning laser to dissipate the charge on the areas of the PIP exposed to the light and developing to form a latent electrostatic image. The resultant latent image is developed by subjecting the latent image to a liquid toner comprising a carrier liquid and colored toner particles. These toner particles are generally comprised of a pigmented polymer. Generally, the development is carried out, at least partially, in the presence of an electric field, such that the toner particles are attracted either to the charged or discharged areas, depending on the charge of the particles and the direction and magnitude of the field.
This image may then be transferred to a substrate such as paper or plastic film, often via an intermediate transfer member (“ITM”) which is typically covered with a replaceable blanket. The transferred image may then be permanently affixed to the substrate by the application of pressure, heat, solvent, overcoating treatment or other affixing processes. In general, in the commercial process used by HP-Indigo, the ITM is heated to a temperature that causes the toner particles and residual carrier liquid to form a film in the printed areas which is transferred to the final substrate by heat and pressure. Fixing to the final substrate takes as part of the transfer process.
There are two basic methods for printing in color using an LEP process. The first method is a 4-shot process. In the 4-shot process, each printed color separation is transferred separately from the ITM to the substrate, until the full color image is achieved. Once the full color image has been deposited onto the substrate, the substrate is passed out of the printer. The second method is called a 1-shot process. In the 1-shot process, the printed colors are transferred one at a time to the ITM. When all the colors have been transferred to the ITM they are transferred together from the ITM to the substrate at the same time, instead of one at a time, like in the 4-shot process. In some applications more than 4 colors are used to form the final image. Color toners that are widely used in the industry include the HP ElectroInk® products which contain colored polymer toner particles and a carrier liquid including a volatile portion, such as Isopar®L. In principle, there is no limit to the number of colors that can be used in either process.
Conventionally, electrophoresis is used to develop an image on a PIP. In a typical electrophoretic printer or copier, a PIP charged to a high voltage is exposed to light in certain regions, producing a latent image in which the voltage is reduced to a lower voltage depending on the exposure at each position. A toner, such as a liquid toner, with toner particles dispersed in a carrier liquid, is placed between the surface of the PIP and a development electrode, electrified to a voltage that is intermediate between the maximum and minimum voltage on the selectively exposed photosensitive layer. The development electrode thus produces an electric field normal to the surface of the PIP which is directed toward the PIP or away from it, depending on the potential at each position which in turn depends on how much light each position was exposed to.
Toner particles in the liquid toner migrate toward or away from the PIP, depending on the direction of the electric field at each position, and as a result, toner particles are selectively deposited on the surface of the PIP, converting the latent image into a developed toner image. For positions that were exposed to an intermediate amount of light, the density of toner particles may depend on the exposure at that position.
Japanese patent publication 50-152741, the disclosure of which is incorporated herein by reference, describes an electrophoretic printer in which liquid toner emerges from an opening in the middle of an electrode, and flows in along a gap between the electrode and a rotating PIP. The toner flows in both direction from the opening, i.e., in the same direction as the rotating cylinder, and in the opposite direction.
Alternatively to the electrophoresis method, a binary image developing technique is used. U.S. Pat. No. 5,596,396 to Landa et al, U.S. Pat. No. 5,610,694, to Lior et al, and PCT application PCT/IL2005/000217 to Kella, the disclosures of which are incorporated herein by reference, describe a development method called binary image development. In binary image development, instead of introducing a freely flowing liquid toner with charged particles against the surface of the PIP, a viscous concentrated layer of charged liquid toner particles coating a developer cylinder of a binary image developer (“BID”) is placed against the surface of the PIP. The developer cylinder is at a voltage intermediate between the maximum and minimum voltage of the PIP. The two cylinders rotate, and different portions of the toner layer progressively come into contact with the PIP at a nip between the two cylinders. Depending on the direction of the electric field between the developer cylinder and the PIP at each point as it passes the nip, portions of the toner layer either are transferred from the developer cylinder to the PIP, or remain on the developer cylinder. This produces a developed toner image on the surface of the PIP, an image that, at each point, is either toned by the toner or left untoned.
Alternatively, as described in U.S. Pat. No. 5,610,694, less than the full thickness of the toner layer is transferred from the developer cylinder to the PIP, at those points where toner is transferred at all. This method may make the resulting developed image on the PIP less sensitive to possible non-uniformity of the toner layer on the developer cylinder.
To produce the layer of concentrated toner on the developer cylinder in the first place, liquid toner is run in a narrow gap between the rotating developer cylinder and an electrode, which produces an electric field which causes toner particles to adhere to the developer cylinder. As each portion of the surface of the development cylinder rotates beyond the end of the electrode, a squeegee removes excess liquid from that portion of the surface, leaving a uniform layer of concentrated toner coating the development cylinder. After each portion of the surface of the developer cylinder passes the nip and transfers part of the layer to the photosensitive member, a cleaning roller or scraper removes the remaining parts of the toner layer from that portion of the surface of the developer cylinder, providing a clean surface so that a uniform layer of toner can be coated on the developer cylinder for the next image as each portion of its surface passes the electrode again.
Japanese patent application number 09086192 (publication number 10282795), the disclosure of which is incorporated herein by reference, describes such an image development system in which a liquid toner flows into the gap between the electrode and the developer cylinder through an opening in the middle of the electrode. The electrode is adjacent to one side of the developer cylinder, whose surface is moving upward on that side. Some of the liquid toner is carried upward with the surface of the developer cylinder, while some of the liquid toner flows downward along the surface of the developer cylinder, moving in a direction opposite to the direction of motion of the surface. In both the upward and downward moving liquid toner, some toner particles migrate to the surface of the developer cylinder under the influence of the electric field produced by the electrode, and adhere to the developer cylinder.
A similar image development system is described as prior art in PCT publication WO 01/92962, the disclosure of which is incorporated herein by reference, but with the electrode below the developer cylinder instead of to its side. Most of the liquid toner coming out of the opening in the middle of the electrode flows along the gap in the direction of motion of the developer cylinder, but some of it flows along the gap in the opposite direction.